The present disclosure relates to pressure transmitter flow meter connections. More specifically, the invention relates to a self-draining head configuration.
Flow measurement of process fluids such as steam and caustic or dangerous fluids can cause many unique challenges, one of which is common when steam is measured in cold ambient environments. When flow of the steam stops, water trapped by surface tension in the differential pressure ports of a transmitter mount head, or near the diaphragm of the transmitter, can freeze and damage the transmitter, head, or flow meter.
In many differential pressure flow applications it is desirable for process fluids to drain away from the transmitter and back into the process conduit when process flow is stopped. The most common scenario with this requirement is water or steam flow in cold ambient environments where shutting down flow could allow trapped water to freeze, expand, and damage sensitive meter components. It is also desirable for applications with flow of corrosive or other dangerous fluids to self-drain during a shutdown. With current designs, mounting the flow meter with the transmitter above the process conduit does not guarantee that process fluid will drain back into the pipe. The passages in the impulse tubes and ports in the current head and/or manifold have diameters that are small enough that the mass of the fluid within may not overcome the surface tension of the fluid, and thus it will remain trapped, even after process flow stops.
A common process fluid that can cause problems in harsh environments, especially those that are subjected to cold temperatures (e.g., cold enough to freeze water), is steam. In cold temperatures, steam condenses to water, and that water can freeze in head passages, and near diaphragms of a pressure transmitter, causing a number of potential issues including damage to the process transmitter and its components, plugging of the process fluid passages, or the like.
One proposed solution for process fluid being trapped near a transmitter diaphragm or within the head or flow meter is to increase the diameters of process fluid passages on a standard head. Another attempt at avoiding damage caused from freezing water is to create an instrument with geometry that allows water to drain away from the transmitter back into the main process line. Such a “straight drain” design does increase the amount of drainage, but water will only drain out of the impulse tubes, not the head of the meter due to small internal passages and cross-drill head geometry. In such a design, damage to the transmitter caused from freezing water has not been fully mitigated. First, traditional equalizer and isolation valves use small diameter passages in order to properly seal. Second, due to the angles of internal passageways within transmitter mount heads, process fluid will only drain if the transmitter is oriented vertically upward, even with increased diameters. Vertical installation is not feasible for many piping installations.
Therefore, currently, the most common method used to protect a flow meter from freeze damage is heat tracing or steam tracing systems. Both of these techniques are expensive to install, operate, and maintain. For example, differential pressure flow is a desirable technology for measuring steam due to its inherent reliability, wide industry acceptance, and high temperature ranges. Approximately 45% of all energy is used to create steam and approximately 70% or more of all steam measurements use differential pressure flow technology. Many of these measurement points are in ambient environments that use heat tracing or steam tracing to prevent the condensate trapped in the primary element head from freezing and damaging the transmitters due to potentially cold ambient air temperatures. During installation, each steam trace installation for a single flow measurement installation can cost on the order or $1500 to $3000. When maintenance and operational costs are included, this cost is significantly higher.